1. Field of the Invention
The present invention relates to an illumination device and a liquid crystal display device. In particular, the invention relates to an illumination device which can illuminate a wide area brightly and uniformly though having only a single light source as well as to a liquid crystal display device using it.
2. Description of the Related Art
Conventionally, front lights of reflection-type liquid crystal display devices employ a unit that is composed of a light source, an intermediate light guide, a light guide plate, a case body that holds those components in an integral manner and has an inner reflective surface, and other components.
FIG. 23A is a perspective view showing the configuration of a liquid crystal display device having such a configuration. FIG. 23B is a plan view of the liquid crystal display device of FIG. 23A. The liquid crystal display device shown in FIGS. 23A and 23B is composed of a liquid crystal display unit 120 and a front light 110 that is disposed on the front side of the liquid crystal display unit 120. Although not shown in detail, the liquid crystal display unit 120 is a reflection-type liquid crystal display unit that performs display by reflecting light coming from the front side. In the liquid crystal display unit 120, a liquid crystal layer is held between a top substrate 121 and a bottom substrate 122 that are opposed to each other. Display is performed by varying the light transmission state by controlling the liquid crystal orientation state of the liquid crystal layer.
The front light 110 is composed of a flat light guide plate 112, a rod-shaped intermediate light guide 113 that is disposed to a side end face 112a of the flat light guide plate, and a light-emitting element 115 that is disposed adjacent to one end face of the intermediate light guide 113. The top surface of the light guide plate 112 is formed with a prism structure in which a plurality of wedge-shaped (in cross-section) projections 114 are arranged parallel with each other. To prevent a moirxc3xa9 pattern, the projections 114 are formed so as to be somewhat inclined with respect to a light-guide-plate-side end face 112a. 
In the front light 110, light that is emitted from the light-emitting element 115 is applied to the side end face 112a of the light guide plate 112 via the intermediate light guide 113 and is thereby introduced into the light guide plate 112. The light is reflected by the prism-shaped top surface of the light guide plate 112 and is thereby changed in traveling direction. As a result, the light is emitted from the bottom surface (see FIG. 23A) of the light guide plate 112 toward the liquid crystal display unit 120.
In portable electronic equipment such as PDAs and portable game machines, the battery drive time greatly influences the ease of use. Therefore, liquid crystal display devices that are used as display sections of such equipment have come to employ, to lower the power consumption of a front light, a single-light front light that is equipped with only a single light-emitting element like the front light 110 shown in FIG. 23A is. That is, it is intended to reduce the power consumption by decreasing the number of light-emitting elements. Further, miniaturization of portable electronic equipment requires that the thickness of the front light 110 be decreased to about 1 mm.
However, in such single-light front lights, it is almost impossible to uniformly illuminate a wide display screen of several inches or more with the combination of the thin light guide plate and the single light-emitting element. More specifically, in the front light 110 shown in FIG. 23A in which the light-emitting element 115 is provided on one side, to uniformly introduce light coming from the light-emitting element 115 into the light guide plate 115, first it is necessary to make the incident light uniform in the longitudinal direction of the side end face 112a of the light guide plate 112 by means of the intermediate light guide 113. However, since making the incident light on the light guide plate 112 uniform by means of the intermediate light guide 113 is difficult in itself, it is very difficult to obtain output light that is uniform over the entire light exit surface of the light guide plate 112. In a serious case, this results in a problem that, as shown in FIG. 23B, a triangular (in a plan view) dark portion 118 may occur close to the light-emitting-element-115-side sideline (left-hand-sideline in FIG. 23B) of the light guide plate 112, which lowers the legibility of the liquid crystal display device.
Where the light guide plate 112 is made thinner to reduce the thickness and size of portable electronic equipment, light traveling inside the light guide plate 112 is prone to leak out of the light guide plate 112 in being reflected by the surfaces of the light guide plate 112. This results in a problem that the light quantity decreases as the position goes away from the light-emitting element 115.
As described above, whereas the demand for the front light using a single light-emitting element as a light source is increasing, no front light has been realized which can illuminate a large area brightly and uniformly though it is thin.
The present invention has been made to solve the above problems, and one object of the invention is therefore to provide an illumination device which can illuminate a large area brightly and uniformly though its power consumption is low.
Another object of the invention is to provide a liquid crystal display device that is equipped with such an illumination device and is high in luminance and superior in display quality.
To attain the above objects, the invention employ the following configurations.
The invention provides an illumination device comprising a light source, and a light guide plate for receiving light emitted from the light source through one side end face thereof, and for outputting the light traveling inside the light guide plate through one major surface thereof as a light exit surface, the other major surface, being a light reflecting surface, of the light guide plate being formed, in stripe form in a plan view, with a plurality of prism grooves each of which is formed by a slight slope and a steep slope having an inclination angle larger than that of the slight slope, the inclination angle xcex81 of the slight slope and the inclination angle xcex82 of the steep slope being in ranges of 1xc2x0 to 10xc2x0 and 41xc2x0 to 45xc2x0, respectively, the width of the steep slope in the reflecting surface increasing as the steep slope goes away from the light incident surface in such a manner that the width of the steep slope amounts to 1.1 to 1.5 at a position farthest from the light incident surface if it is assumed that the width of the steep slope is equal to 1.0 at a position closest to the light incident surface.
According to the above configuration, since the shapes and the dimensions of the prism grooves which are formed in the reflecting surface of the light guide plate are set within the above ranges, a high-luminance illumination device can be obtained in which the exit light quantity is uniform in the light exit surface of the light guide plate and the efficiency of utilization of the light source is high. If the inclination angle xcex81 of the slight slope is smaller than 1xc2x0, the desired amount of luminance for the illumination device is not obtained. If it is larger than 10xc2x0, the uniformity of the quantity of exit light that is output from the light exit surface of the light guide plate lowers. Therefore, such ranges of the inclination angle xcex81 are not preferable. If the inclination angle xcex82 of the steep slope is smaller than 41xc2x0 or larger than 45xc2x0, the luminance of the illumination device lowers. Therefore, such ranges of the inclination angle xcex82 are not preferable.
The width of the steep slope is measured in the direction perpendicular to the extending direction of the prism grooves. The proportion of light that is reflected by the prism groove to go down increases as the width of the steep slope increases. Therefore, in the illumination device having the above configuration, the width of the steep slope is set at a proper value in accordance with the distance from the light incident surface of the light guide plate. More specifically, in a region that is close to the light incident surface where the quantity of light traveling inside the light guide plate is relatively large, the width of the steep slope is made relatively small, whereby the proportion of light that is reflected to go down is decreased. Conversely, in a region that is distant from the light incident surface, the width of the steep slope is made large, whereby the proportion of light that is reflected to go down is increased. In this manner, the distribution of the exit light quantity is made uniform over the entire light guide plate. In the invention, the width of the steep slope amounts to 1.1 to 1.5 at a position farthest from the light incident surface if it is assumed that the width of the steep slope is equal to 1.0 at a position closest to the light incident surface, whereby the distribution of the exit light quantity is made uniform. As will be described later in the examples, the present inventors have confirmed that the above setting ranges of the inclination angles of the slopes and the width of the steep slope are proper.
The illumination device according to the invention may be such that the width of the steep slope of each of the prism grooves is approximately constant in the prism groove extending direction, and that the width of the steep slope increases as the prism groove average distance from the light incident surface increases.
More specifically, in the illumination device according to the invention, the prism grooves are formed in such a manner that the width of the steep slope gradually increases as the steep slope goes away from the light incident surface starting from the steep slope closest to the light incident surface. The width of the steep slope farthest from the light incident surface is 1.1 to 1.5 times the width of the steep slope closest to the light incident surface. With this structure, the proportion of light that is reflected by the steep slope to go down increases as the prism groove goes away from the light incident surface of the light guide plate. As a result, the exit light quantity can be made uniform in the light exit surface of the light guide plate.
The illumination device according to the invention may be such that the extending direction of the prism grooves crosses the light incident surface.
Where an object to be illuminated by the illumination device has periodic shapes or patterns that are arranged at prescribed intervals, this structure prevents occurrence of a moirxc3xa9 pattern due to optical interference between the prism grooves of the light guide plate and the shapes and patterns of the object to be illuminated. For example, where the object to be illuminated is a liquid crystal display unit, optical interference between the prism grooves and a matrix-like pixel arrangement would otherwise cause a problem. The above structure prevents occurrence of such a moirxc3xa9 pattern and provides superior legibility.
The illumination device according to the invention may be such that the width of the steep slope of each of the prism grooves increases continuously in the prism groove extending direction as the position goes away from the light incident surface.
With this structure, in the case where the prism grooves are formed so as to cross the light incident surface, the width of the steep slope can be increased as the position goes away from the light incident surface. Therefore, the proportion of light that is reflected by the steep slope to go down can be increased as the position goes away from the light incident surface of the light guide plate. As a result, the exit light quantity can be made uniform in the light exit surface of the light guide plate.
In the illumination device according to the invention, it is preferable that the inclination angle xcex1 of the prism grooves that is formed by the extending direction of the prism grooves and the longitudinal direction of the light incident surface be larger than 0xc2x0 and smaller than or equal to 15xc2x0. It is even more preferable that the inclination angle xcex1 of the prism grooves be in a range of 6.5xc2x0 to 8.5xc2x0.
With this structure, the uniformity of the exit light quantity in the light exit surface of the light guide plate can be increased. Further, the optical interference between an object to illuminated and the light guide plate can be suppressed more efficiently. This advantage is greater in the case where the object to illuminated has periodic shapes or patterns that include straight portions parallel with the light incident surface of the light guide plate and whose repetition direction is perpendicular to the light incident surface of the light guide plate.
The above advantage cannot be obtained if the inclination angle xcex1 of the prism grooves is equal to 0xc2x0 or larger than 15xc2x0. Setting the inclination angle xcex1 of the prism grooves within the range of 6.5xc2x0 to 8.5xc2x0 provides an illumination device with a uniform exit light quantity distribution and that is not prone to cause a moirxc3xa9 pattern due to interference with an object to be illuminated.
In the illumination device according to the invention, the light source may comprise an intermediate light guide consisting of two light guide members that extend along the light incident surface of the light guide plate and a side end face of the light guide plate connecting to the light incident surface, respectively, and that are arranged so as to assume an L shape in a plan view, and a light-emitting element that is disposed adjacent to an end face of at least one of the two light guide members.
This structure makes it possible to supply light through two side end faces of the light guide plate. Since more light can be supplied to the light-emitting-element-side end portion of the light guide plate where the exit light quantity is prone to decrease in conventional single-light illumination devices, the uniformity of the exit light quantity in the light exit surface of the light guide plate is improved.
In the illumination device according to the invention, the light source may comprise an intermediate light guide extending along the light incident surface of the light guide plate and a light-emitting element disposed adjacent to an end face of the intermediate light guide, and a metal reflection film may be formed on a side end face of the light guide plate that connects to the light incident surface.
With this structure, since the reflection film is formed on the side end face of the light guide plate that connects to the light incident surface, leakage of light through, in particular, the light-emitting-element-side end face of the light guide plate can be prevented. Since light that is reflected by this reflection film returns to the light guide plate, decrease in exit light quantity in, in particular, the light-emitting-element-side end portion of the light guide plate can be prevented effectively. As a result, a highly uniform distribution of the exit light quantity can be obtained.
In the illumination device according to the invention, the light source may comprise an intermediate light guide extending along the light incident surface of the light guide plate and a light-emitting element disposed adjacent to an end face of the intermediate light guide, and the light guide plate may project, in the longitudinal direction of the light incident surface, from a display region of the light guide plate through which an object to be illuminated by the illumination device is to be transmission-displayed.
In this illumination device, by projecting the light guide plate in the extending direction of the intermediate light guide, the light-emitting-element-side portion of the light guide plate particularly where the exit light quantity is prone to decrease is excluded from the display region. As a result, an illumination device in which the exit light quantity is substantially uniform can be provided. The display region of the illumination device according to the invention is a region of the light guide plate through which an object to be illuminated that is disposed on the back side of the illumination device is transmission-displayed, and that substantially corresponds to the whole area (in a plan view) or a display area of the object to be illuminated.
The illumination device according to the invention may be such that the projection width xcex94W of the light guide plate, the inclination angle xcex1 of the prism grooves, and the length L of a side end face of the light guide plate connecting to the light incident surface satisfy a relationship xcex94Wxe2x89xa7Lxc3x97tan xcex1.
In this illumination device, since the extension length of the light guide plate is set so as to satisfy the above formula, a dark portion that may occur in the light-emitting-element-side portion of the light guide can reliably be located outside the display region of the light guide plate. Therefore, an illumination device can be provided that is superior in the uniformity of the exit light quantity in the display region.
In the illumination device according to the invention, it is preferable that the outer side face of the intermediate light guide be a prism surface that is formed with a plurality of wedge-shaped (in cross-section) grooves, and that a reflection film be formed on the prism surface.
Light that has entered the intermediate light guide from the light-emitting element is reflected by the prism surface in traveling inside the intermediate light guide, is output from the surface that is opposite to the prism surface, and then enters the light guide plate. With the above structure, the reflection film formed on the prism surface increases the reflectivity of the prism surface and can thereby increase the quantity of light that is reflected toward the light guide plate. As a result, the quantity of light entering the light guide plate is increased, which in turn increases the luminance of the illumination device.
Next, a liquid crystal display device according to the invention comprises any of the illumination devices described above and a liquid crystal display unit that is illuminated by the illumination device.
Provided with the illumination device capable of illuminating a large area uniformly at high luminance, this liquid crystal display device can provide superior display quality having high luminance and a uniform brightness distribution. The brightness uniformity is not lowered though the illumination device has only a single light-emitting element. This makes-it possible to provide a liquid crystal display device that is superior in display quality and low in power consumption.
The liquid crystal display device according to the invention may be such that the illumination device is disposed on the front side of the liquid crystal display unit, and that the prism grooves of the light guide plate have a pitch P1 that is in a range of (xc2xd)P0 less than P1 less than (xc2xe)P0, where P0 is a pixel pitch of the liquid crystal display unit.
By setting the pixel pitch P0 of the liquid crystal display unit and the pitch P1 of the prism grooves so that they satisfy the above relationship, optical interference between those periodic structures can be prevented. This prevents deterioration in the legibility of the liquid crystal display device due to a moirxc3xa9 pattern that would otherwise be caused by such interference.
If the pitch P1 of the prism grooves is smaller than (xc2xd)P0 or larger than (xc2xe)P0, a moirxc3xa9 pattern is prone to occur due to interference between the two structures.
In the liquid crystal display device according to the invention, it is preferable that the angle xcex2 that is formed by the extending direction of the prism grooves and pixel arrangement direction of the liquid crystal display unit that is parallel with the light incident surface of the light guide plate is larger than 0xc2x0 and smaller than or equal to 15xc2x0. It is even more preferable that the angle xcex2 is in a range of 6.5xc2x0 to 8.5xc2x0.
The angle xcex2 is an inclination angle of the prism grooves with respect to the pixel arrangement direction. If the inclination angle xcex2 is 0xc2x0, it is difficult to obtain the effect of preventing a moirxc3xa9 pattern. If the inclination angle xcex2 is larger than 15xc2x0, the display luminance is low and a problem arises that a large dark portion appears in the light guide plate of the illumination device. Setting the inclination angle xcex2 within the range of 6.5xc2x0 to 8.5xc2x0 makes it possible to prevent the optical interference almost completely and thereby realize a liquid crystal display device that is much superior in visibility. The angle xcex2 that is formed by the prism grooves and the pixel arrangement direction of the liquid crystal display unit is equal to the above-mentioned inclination angle xcex1 in the case where the pixel arrangement direction of the liquid crystal display unit is parallel with the light incident surface of the light guide plate of the illumination device.
The liquid crystal display device according to the invention may be such that the liquid crystal display unit comprises a top substrate and a bottom substrate that are opposed to each other, liquid crystal molecules that have positive dielectric anisotropy and are held between the top and bottom substrates, and a reflection layer and color filters that are laid on the inner surface of the bottom substrate, the reflection layer having a reflection characteristic that reflection luminance is approximately constant in a reflection angle range of 10xc2x0 to 40xc2x0 with respect to the normal to a display surface of the liquid crystal display unit.
This structure enables a display in which the luminance is constant in a wide view angle range, and hence makes it possible to realize a liquid crystal display device that can be used comfortably.
In the liquid crystal display device according to the invention, the reflection layer may have a reflection characteristic that a reflection luminance curve is approximately symmetrical with respect to a specular reflection angle of incident light. This structure makes it possible to provide a liquid crystal display device in which reflection light is diffused in a prescribed angle range with respect to a specular reflection angle of incident light.
In the liquid crystal display device according to the invention, the reflection layer may have a reflection characteristic that a reflection luminance curve is not symmetrical with respect to a specular reflection angle of incident light. This structure can increase luminance of reflection light (display light) in a prescribed direction. Even where, for example, the liquid crystal display device is used in a state that a user is not in the specular reflection direction of the liquid crystal display device, sufficient luminance can be secured in the direction of the user. A liquid crystal display device can be obtained that can be used more comfortably.
In the liquid crystal display device according to the invention, the liquid crystal display unit may be an active matrix liquid crystal display unit or a passive matrix liquid crystal display unit. In either liquid crystal display device, since the illumination device according to the invention is disposed on the front side of the liquid crystal display unit, a high-quality display can be obtained that has a uniform brightness distribution over the entire display area and is free of a moirxc3xa9 pattern.